Known in the art is a tennis ball projector (cf. USSR Inventor's Certificate No. 577,044, Int. Cl. A 63 B,69/40, "Discoveries, Inventions, Industrial Designs and Trademarks" bulletin No. 39, 1977), comprising a chamber communicating with a source of compressed air.
There is also a ball projection barrel with an outlet end face thereof protruding from the chamber and an inlet end face thereof located in the chamber to admit compressed air therefrom.
The barrel has a side hole wherethrough the barrel communicates with a gate and a branch pipe to admit balls into the gate.
Provision is also made of a valve for periodic communication between the barrel and the chamber, installed close to the chamber-enclosed barrel end face.
The valve is made in the form of a rod-mounted disk interacting with the barrel end face. A spring presses the disk to the barrel inletend face. The valve rod is arranged along the barrel axis in a bushing secured in the chamber cover.
The valve is actuated by an electromagnet with a movable core kinematically associated with the free end of the valve rod by means of a lever. As the core is pulled in or out, the valve is opened or closed respectively. With the valve open, the compressed air is free to enter the barrel and project the ball.
This prior art tennis ball projector may be used effectively for projecting balls at a relatively low speed due to the small effort of the electromagnet to open the valve intended for admission of compressed air into the barrel.
However, to project a ball at a high initial velocity, e.g. to imitate a champion's shot, the effort exerted by the electromagnet must be as great as 400-500N. Such an effort could be attained by increasing considerably the mass of the movable parts of the tennis ball projector and for the electromagnet, which would appreciably augment the weight of the projector and extend essentially the valve opening time.
Slow opening of the valve results in a low efficiency of the tennis ball projector, because the valve opening time is longer than that required for the ball to pass through the barrel. Therefore, a relatively weak flow of compressed air, not a powerful jet of it, enters the barrel, due to which a considerable amount of air is ejected from the barrel after the ball has been shot out. In other words, this amount is not spent for the purpose it is intended for, i.e. for projecting the ball.